This invention relates to sliding vane pumps.
Sliding vane pumps are typically used to provide hydraulic pressure and flow to various types of hydraulic systems, such as hydraulic power assist steering systems in automobiles. One example of a common sliding vane pump includes a rotor eccentrically mounted in a cylindrical chamber. As the rotor rotates, vanes within the rotor slide in and out to follow the contour of the housing, pushing fluid from an inlet port to an outlet port in the process.
Another style is referred to as a hydraulically balanced sliding vane pump, which uses a rotor configuration such as that shown in FIG. 2. In this configuration, rotor 12 is centrally located in an oblong, or elliptical chamber 15 defined by pump ring 13. Chamber 15 includes two inlets 16 and two outlets 18, with rotor 12 rotating counter-clockwise as shown. Vanes 22 are radially disposed in radial slots 24. Under the influence of fluid pressure from down stream of the outlets 18, vanes 22 are urged out of slots 24 to follow the contour of chamber 15. Vanes 22 therefore urge fluid along in spaces, or pumping cavities, between the vanes from the inlets 16 to outlets 18 as rotor 12 rotates.
The function of the pumping cavity is to transfer a discrete volume of fluid at low pressure to high pressure. This happens repeatedly during a rotation of the pump shaft due to the presence of multiple pumping cavities. The end result is a steady flow of fluid discharged from the discharge port. Ideally for this to occur, the volume of fluid in the pumping cavity will be compressed and just reach the particular discharge pressure as it is allowed to enter the discharge port, providing a smooth transition from low to high pressure. However, this is seldom the case in practice. During operation, the pressure of the fluid in the pumping cavity is not the same as the pressure of the fluid in the discharge port just prior to the leading vane passing the discharge port. If the pressure in the cavity is lower than the pressure in the discharge port, fluid will quickly flow into the pumping cavity as the leading vane passes the discharge port. Conversely, if the pressure in the pumping cavity is higher than that in the discharge port, then fluid will quickly flow out of the pumping cavity as the leading vane passes the discharge port. This flow pulse is superimposed upon the steady flow of oil discharged from the discharge port. This small but quick flow pulse results in a corresponding pressure pulse (positive or negative) in the discharge port when the leading vane passes the discharge port.
Since the pressure pulse occurs every time the leading vane passes the discharge port, the pulse occurs at vane passage frequency. Since there are multiple vanes passing the discharge port during one revolution of the pump shaft, and the pump shaft is rotated at a constant speed, the vane passage frequency will be an integer multiple of the pump shaft rotation frequency.
The pressure pulse acts upon components within the pump, and components located downstream of the pump, causing these components to vibrate at the corresponding frequency of the pulse. Vibration of these components can radiate sound that is undesirable.
The annoyance of pump noise is due not only because it is loud, but also because it is tonal in nature, due to the repeating of discrete pumping cycles, which occur with equal time intervals between them, every time a vane passes the outlet ports of the pump.
Another drawback to sliding vane hydraulic pumps is that their speed range is limited by cavitation of the fluid within the pumping chamber. Cavitation is the formation and collapse of low-pressure bubbles in liquids. These bubbles are caused by air or vapor absorbed or otherwise entrained in the hydraulic fluid. Cavitation greatly increases pressure ripple which causes excessive noise and vibration, as well as loss of performance. The use of a jet supercharger to increase the inlet pressure to the pump has been used to increase the speed at which cavitation becomes audible, but these efforts have not sufficiently increased cavitation speed for many applications. Another approach has been to remove the air from the hydraulic fluid, but this has proven to be difficult in practice. Yet another drawback is that sliding vane pumps have a fixed capacity, i.e., they pump a fixed amount of fluid in each revolution of the rotor. This is a serious drawback of this type of pump in certain applications. For example, in the automotive industry, the hydraulic pump is often driven by an internal combustion engine that operates at a speed independent of the needed hydraulic power.
The above-listed drawbacks and disadvantages of the prior art sliding vane pumps are overcome or alleviated by a high speed dual discharge sliding vane hydraulic pump with two external discharge ports for varying the capacity of the pump and/or irregularly spaced vanes to reduce the tonal characteristics of noise caused by pressure ripple effects and/or increasing the inlet slot length to increase the cavitation speed.